Industrial robots usually comprise a robot foot, a stand and a robot arm. The stand is rotatably arranged on the robot foot. The robot arm is rotatably arranged in a joint on the stand. A six axis industrial robot, for example, comprises a first robot arm, a second robot arm, a wrist arranged with a tool holder and drive means comprising an electrical motor and a reduction gear rotation of each axis.
In its initial position, which may also be a rest position, the robot is oriented with the first arm approximately vertical. When the robot is moving/operating, the first arm is rotated in relation to the stand while at the same time the first and second arm rotate in relation to each other. The total moving mass consists of the handling weight applied to the wrist as well as the actual dead weight of the robot. Upon movement of the robot, the drive means concerned rotates the robot arms from the rest position/initial position as defined above, whereby the gravitational force acting on the arms generates a torque.
The expression rotation from a rest position/initial position in this context means a rotation in a direction where the gravitational force contributes to the rotation of a robot arm. This rotation is called balancing in the following. A balancing device is a device so constructed that, during the rotation from the rest position, it generates a torque, which acts to return the robot to its rest position/initial position. Thus, a balancing device assists/relieves the activated drive motor/motors during the lifting/rotation back.
The expression rotation back to the rest position/initial position thus means a rotation that counteracts and hence compensates for the gravitational force.
A commonly used balancing device comprises a balancing cylinder, a piston, elastic spring means and a pull rod. During operation of a robot comprising a balancing cylinder, the pull rod is arranged to move out or in of the cylinder to compress or release the spring unit arranged within the cylinder. Thereby energy is stored within or is released from the spring unit.
A balancing cylinder, as mentioned above, comprises a cavity inside the cylinder. This cavity is filled with grease and air. The cylinder has at least one opening, that is where the pull rod is moving in and out and this opening is sealed. If there is another opening in the cylinder for some reason, it is also sealed.
The pull rod of an unloaded balancing cylinder occupies a small volume inside the cylinder. Every time the rod is moved out during operation, a vacuum will arise inside the cylinder. If a robot is operating in an environment with high humidity, there is a risk that moisture is sucked in through any sealing due to this vacuum.
The same problem arises when a robot is working in an application where there is a hundred percentage of humidity. Then, there is a risk that liquid on sealing surfaces is sucked in due to this vacuum.
During operation of an industrial robot, the temperature is increasing within the robot. Additionally, the environment can be warm and humid. An increasing temperature during operation of a robot causes air within a balancing cylinder to expand its volume. Finally, when the robot is shut down, the temperature is dropping, which causes the internal air volume of the balancing cylinder to be reduced and a vacuum will arise. When this occurs in a balancing cylinder comprising a worn sealing, there is even a risk that liquid is pouring into the cylinder.
It is most important to avoid moist or liquid inside the cylinder of a balancing device since it causes corrosion of the complete balancing cylinder. Corrosion within a balancing cylinder results in malfunction, which causes a negative impact on the life time of a spring balancing device.
JP9085674 teaches to cool down a drive motor for an articulated robot without installing a cooling fan by providing a balancer having air holes opened thereto on its both sides and connecting the air holes to a blast pipe opened toward side faces of the drive motor. A piston 43 compresses a compression spring 42 and at the same time exhausts the air between the piston 43 and the top part 412 of a cylinder 41 to the side face of a drive motor 21 via an air hole 46 and a blast pipe 48 so as to cool down the drive motor 21 (FIG. 6).
JP04372383 teaches an industrial robot comprising a robot arm and a balance device with a piston-cylinder mechanism comprising a spring means. The cylinder housing comprises an air inlet through which pressurized air is supplied to the spring housing. The robot arm is balanced with a total sum of air pressure power and spring power in the entire region where it can be rotated.
Thus, there are needs to increase the life time of a spring balancing unit comprised in an industrial robot working in environments with high humidity.
These needs cannot be fulfilled by the balancing systems according to the above-mentioned prior art.